The present invention relates to a supply circuit with inverter for discharge lamps. More particularly the present invention relates to a supply circuit for discharge lamps with heated electrodes, in which an inverter comprising a set of electronic switches, turned on and off alternately, supply a load circuit having at least one lamp and an LC resonant circuit in series with the lamp.
Systems for controlling the voltage on the electrodes of a discharge lamp and which have the objective of modifying the behavior of the load circuit or of turning off the supply thereto in case of defective operation of the lamp, are often used in supply circuits of the type mentioned above, with the objective of preventing excessively high voltages from arising between the electrodes.
From European Patent Application EP-A-0 610 642, there is known a supply circuit with an inverter for discharge lamps, in which associated with the load circuit is a control circuit comprising a voltage-dependent resistor (VDR) in series with a dissipative element. When the voltage at one terminal of the VDR exceeds a threshold value (which occurs in the case of failure of the lamp to light following a defect therein, for example), the VDR becomes conducting with the consequence that the resonant circuit in series with the lamp receives an additional dissipative element. This modifies the quality factor of the circuit and hence reduces the voltage at the terminals of the lamp. Provision is further made for a timer circuit which turns off the supply to the load circuit should the overvoltage condition last for a time greater than a pre-set threshold value.
From European Patent Application EP-A-0 113 451, there is known a different overvoltage control system, in which a voltage-dependent resistor in series with a capacitor are inserted in parallel with a branch of the load circuit. In this case, when a voltage difference greater than a specified threshold value is generated between the terminals of the VDR, it becomes conducting and inserts an auxiliary capacitor into the load circuit, modifying the frequency of resonance of the resonant circuit in series with the lamp.
Traditional circuits for protection from overvoltages come into operation when the voltage between the electrodes of the lamp exceeds a threshold value. In the case of a defective lamp, and hence of the failure of this lamp to light, the voltage between the electrodes of the lamp reaches values of the order of 1000 V. Conversely, when the lamp is removed from the load circuit, the potential difference between the electrodes is of the order of 700 V. The circuits currently available are unable to discriminate between these two voltage values, which may moreover vary from one instance to another of the circuit. Consequently, the overvoltage circuits cut in anyway, turning off the supply with a pre-specified time (of the order of 300 ms) having elapsed from the onset of the establishment of a situation of overvoltage between the electrodes of the lamp. They are further unable to distinguish between the two conditions of a defective lamp and a removed lamp.
On the other hand, it would be appropriate to make provisions for a circuit which is able to discriminate between a situation of actual defective operation and a situation where a lamp is removed from the load circuit in such a way that the substitution of the lamp does not entail the disabling of the supply inverter of the load circuit. Thus, in currently known circuits, the disabling of the supply inverter is permanent and hence requires the intervention of the operator in order to reactivate the supply for the lamp, even when the disabling has occurred through simple substitution of the lamp rather than through a defect in operation thereof. On the other hand, a high current flows in the case of overvoltage due to a defect in the operation of the lamp in the load circuit which passes through the capacitor in parallel with the lamp. This anomalous condition may lead to the over-stressing of the inverter and hence to damage thereto. Protection from overvoltage has the objective of preventing this consequence. Conversely, when the voltage in the load circuit increases on account of the lamp being removed, the current which flows in the circuit is practically zero, and hence the inverter does not experience the dangerous stresses which occur under conditions of a faulty lamp.